1. Field of the Invention
This invention relates to glassy metal alloys with near-zero magnetostriction and high saturation induction.
2. Description of the Prior Art
Saturation magnetostriction .lambda..sub.s is related to the fractional change in length .DELTA.l/l that occurs in a magnetic material on going from the demagnetized to the saturated, ferromagnetic state. The value of magnetostriction, a dimensionless quantity, is often given in units of microstrains (i.e., a microstrain is a fractional change in length of one part per million).
Ferromagnetic alloys of low magnetostriction are desirable for several interrelated reasons:
1. Soft magnetic properties (low coercivity, high permeability) are generally obtained when both the saturation magnetostriction .lambda..sub.s and the magnetocrystalline anisotropy K approach zero. Therefore, given the same anisotropy, alloys of lower magnetostriction will show lower dc coercivities and higher permeabilities. Such alloys are suitable for magnetostatic shields or various other low frequency magnetic applications.
2. Magnetic properties of such zero magnetostrictive materials are insensitive to mechanical strains, provided the material is in the glassy state. When this is the case, there is no need for stress-relief annealing after winding, punching or other physical handling needed to form a device from such material. In contrast, magnetic properties of stress-sensitive materials, such as the crystalline alloys, are seriously degraded by such cold working and must be carefully annealed.
3. The low dc coercivity of zero magnetostrictive materials carries over to ac operating conditions where again low coercivity and high permeability are realized (provided the magnetocrystalline anisotropy is not too large and the resistivity not too small). Also because energy is not lost to mechanical vibrations when the saturation magnetostriction is zero, the core loss of zero magnetostrictive materials can be quite low. Thus, zero magnetostrictive magnetic alloys (of moderate or low magnetocrystalline anisotropy) are useful where low loss and high ac permeability are required. Such applications include a variety of tape-wound and laminated core devices, such as power transformers and signal transformers.
4. Finally, electomagnetic devices containing zero magnetostrictive materials generated no acoustic noise under ac excitation. While this is the reason for the lower core loss mentioned above, it is also a desirable characteristic in itself because it eliminates the hum inherent in many electromagnetic devices.
There are three well-known crystalline alloys of zero magnetostriction (in atom percent, unless otherwise indicated):
1. Nickel-iron alloys containing approximately 80% nickel ("80 nickel permalloys");
2. Cobalt-iron alloys containing approximately 90% cobalt; and
3. Iron-silicon alloys containing approximately 6wt.% silicon.
Also included in these categories are zero magnetostrictive alloys based on the binaries but with small additions of other elements such as molybdenum, copper or aluminum to provide specific property changes. These include, for example, 4% Mo, 79% Ni, 17% Fe (sold under the designation Moly Permalloy) for increased resistivity and permeability; permalloy plus varying amounts of copper (sold under the designation Mumetal) for magnetic softness and improved ductility; and 85 wt.% Fe, 9 wt.% Si, 6 wt.% Al (sold under the designation Sendust) for zero anisotropy.
The alloys included in (1) are the most widely used of the three classes listed above because they combine zero magnetostriction with low anisotropy and are, therefore, extremely soft magnetically; that is they have a low coercivity, a high permeability and a low core loss. These permalloys are also relatively soft mechanically so that they are easily rolled into sheet form, cut into tape form, and stamped into laminations. However, these materials have saturation inductions (B.sub.s) ranging only from about 6 to 8 kGauss, which is a drawback in many applications. For example, if a given voltage V is required at the secondary of a signal transformer or a power transformer, then Farady's law, V .varies. -NA.DELTA.Bf, shows that for a fixed frequency "f" and number of secondary turns N, the cross-sectional area A of core material may be reduced if a larger change in flux density .DELTA.B can be had by using a material of greater B.sub.s. The use of less core material obviously reduces the size, weight and cost of the device as well as reducing both the amount of wire needed to obtain N winding turns and the loss in that wire.
2. Alloys based on Co.sub.90 Fe.sub.10 have a much higher saturation induction (B.sub.5 about 19 kGauss) than the permalloys. However, they also have a strong negative magnetocrystalline anisotropy, which prevents them from being good soft magnetic materials. For example, the initial permeability of Co.sub.90 Fe.sub.10 is only about 100 to 200.
3. Fe/6 wt% Si and the related ternary alloy Sendust (mentioned above) also show higher saturation inductions (B.sub.s about 18 kGauss and 11 kGauss, respectively) than the permalloys. However, these alloys are exteremely brittle and have, therefore, found limited use in powder form only.
Clearly desirable is a zero magnetostrictive alloy of higher saturation induction than the permalloys but retaining low magnetic anisotropy and good ductility.
It is known that magnetocrystalline anisotropy is effectively eliminated in the glassy state. It is, therefore, desirable to seek glassy metal alloys of zero magnetostriction. Such alloys might be found near the compositions listed above. Because of the presence of metalloids which tend to quench the magnetization by the transfer of charge to the transition-metal d-electron states, however, glassy metal alloys based on the 80 nickel permalloys are either non-magnetic at room temperature or have unacceptably low saturation inductions. For example, the glassy alloy Fe.sub.40 Ni.sub.40 P.sub.14 B.sub.6 (the subscripts are in atom percent) has a saturation induction of about 8 kGauss, while the glassy alloy Ni.sub.49 Fe.sub.29 P.sub.14 B.sub.6 Si.sub.2 has a saturation induction of about 4.6 kGauss and the glassy alloy Ni.sub.80 P.sub.20 is non-magnetic. No glassy metal alloys having a saturation magnetostriction approximately equal to zero have yet been found near the iron-rich Sendust composition. Two zero magnetostrictive glassy metal alloys based on the Co-Fe crystalline alloy mentioned above in (2) have been reported in the literature. These are Co.sub.72 Fe.sub.3 P.sub. 16 B.sub.6 Al.sub.3 (AIP Conference Proceedings, No. 24, pp. 745-746 (1975)) and Co.sub.71 Fe.sub.4 Si.sub.15 B.sub.10 (Vol. 14, Japanese Journal of Applied Physics, pp. 1077-1078 (1975)). Table I lists some of the magnetic properties of these materials.
TABLE I ______________________________________ Co.sub.72 Fe.sub.3 P.sub.16 B.sub.6 Al.sub.3 Co.sub.71 Fe.sub.4 Si.sub.15 B.sub.10 ______________________________________ B.sub.s (kGauss) 6.0 6.4 H.sub.c (as quenched)(O.sub.e) 0.023 0.01 B.sub.r (as quenched)(kGauss) 2.84 2.24 H.sub.c (field annealed)(O.sub.e) 0.013* 0.015** B.sub.r (field annealed)(kGauss) 4.5* 5.25** T.sub.C (.degree. K.) 650.degree. 688.degree. ______________________________________ *field annealed at 270.degree. C. for 45 min in 30 Oe applied longitudinally. **field annealed at 350.degree. C. and cooled at 175.degree. C./hr in 400 Oe applied longitudinally.
These glassy alloys show low coercivities and are expected to have high permeabilities and low core loss, because the saturation magnetostriction approximately is zero and, generally, in a glassy state the magnetocrystalline anisotropy is very small and the resistivity is high. However, their saturation inductions are at the lower limit of the range spanned by various high-nickel crystalline alloys. Thus, they offer little improvement over the properties of the crystalline permalloys.